1. Field of the Invention
The present invention is related to the field of hearing aids, and more specifically to the field of permanent cochlear implants for the hearing impaired.
2. Description of the Related Art
Hearing aids are often designed to amplify sound. These do not work, however, for patients with severe deafness because they cannot hear any sound, no matter how loud. For such patients, an implant technology is used, which stimulates directly the cochlea of the human ear.
The implant technology is now described in more detail with reference to FIG. 1. A human pinna (outer portion of the ear) 20 is supported on tissue 22 of the patient's head. When a sound 24 in the audible range of 20 Hz to 20,000 Hz reaches the pinna 20, the sound 24 reaches the eardrum 28 through an opening 26 in the tissue 22, known as the ear canal 26. In a healthy person, the eardrum 28 in turn excites the cochlea 30 mechanically, through additional structures (bones of hearing; not shown). The cochlea 30 accordingly stimulates the hearing nerve 31. In a severely deaf person, however, the cochlea 30 does not produce nerve stimulations, and thus the hearing nerve 31 transmits nothing to the brain.
A cochlear implant has two main parts, an internal unit 32 and an external unit 50. The internal unit 32 is surgically implanted near the pinna 20 of the patient. The internal unit 32 has at least one electrode 34 that is coupled with the cochlea 30. The internal unit 32 includes an internal coil assembly 40 for electrically driving the electrode 34. The internal unit 32 also includes a magnet 42.
The external unit 50 also includes a magnet 52, for suspending the external unit 50 from the field of the magnet 42. The external unit 50 also includes a microphone and signal processing component 54, and an external coil assembly 56. When the sound 24 reaches the microphone and signal processing component 54, the component 54 generates a signal which drives the external coil assembly 56. The signal is then inductively coupled (“injected”) into the internal coil assembly 40.
Referring to FIG. 2, component 54 includes an oscillator 57, which generates a carrier analog signal. A volume control unit 58 is preferably implemented by an adjustable gain amplifier that amplifies the analog carrier signal. Adjusting the volume control unit 58 determines how much overall sound sensation, also known as percept, will reach the patient. In an older implementation of this cochlear implant, the amplified analog carrier signal is input into an amplitude modulator 60, to be amplitude modulated by the received sounds.
Component 54 also includes a microphone 62 with its associated circuitry, a preamplifier 64, and a band pass filter 66. The band pass filter 66 allows to pass only those frequencies of the preamplified electrical signal that correspond to sound in the range of 200 Hz-4000 Hz. The band pass filter 66 discards the signal corresponding to sound 24 that corresponds to the rest of the frequencies, that is, anything outside the frequency range of the band pass filter 66.
The filtered signal from band pass filter (BPF) 66 is input in a modulation control unit 68. The modulation control unit 68 thus uses the filtered signal to modulate the pre-amplified analog carrier signal in modulator 60. The modulator 60 output the modulated signal to an output amplifier 70. Thus amplified, the modulated signal is input into external coil assembly 56 of FIG. 1.
As described immediately above, the prior art limits how much sound percept can be delivered to the patient. This is now explained in more detail.
Referring also to FIG. 3, the interrelationship of the chosen frequencies is placed in perspective. A healthy ear can hear sounds between 20 Hz and 20 kHz (thus these are called the sonic, or hearable frequencies), which is denoted as a sonic range 78. The frequencies important to the understanding of speech belong within a speech range 80, reaching up to a frequency of 8 kHz. The carrier analog signal is denoted in FIG. 3 as an arrow 76, and is at a frequency of 16 kHz, which is within the sonic range 78.
Experiments using the above system showed that sound frequencies higher than 4,000 Hz sounded unclear to the user. Accordingly, the prior art included the band pass filter 66 (seen in FIG. 2), which has a BPF range 86 (seen in FIG. 3).
The result is that the prior art system only works for sounds within the BPF range 86 of the band pass filter 66. The prior art has rationalized this by concluding that the important portion of the sensitivity range of the human ear 78 is the portion where the speech range 80 belongs, and that further, the most important portion of the speech range 80 is found at 4,000 Hz and below.
Patients desire to be able to hear speech at the frequency higher than 4,000 Hz, and other sounds at frequencies higher than 8,000 Hz, just like healthy people.